1. Field of the Invention
The present invention is directed generally to pneumatic brake actuators, and more specifically to a pneumatic brake actuator with a two-way control valve.
2. Description of Related Art
A pneumatic brake system for a large, heavy-duty vehicle such as a bus, truck, semi-tractor, or trailer typically includes a brake shoe and drum assembly which is actuated by an actuator that is operated by the selective application of compressed air. Conventional pneumatic spring brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which actuates the brakes when air pressure has been released from a pressure chamber. The emergency brake actuator, or spring brake, includes a strong compression spring which applies the brake when air is released.
There are two main types of pneumatic brake actuators, piston type actuators and diaphragm type actuators. In the diaphragm type brake actuator, two pneumatic diaphragm brake actuators are typically arranged in a tandem configuration, which includes a pneumatic service brake actuator for applying the normal operating brakes of the vehicle, and a spring brake actuator for applying the parking or emergency brakes of the vehicle. Both the service brake actuator and the spring brake actuator include a housing having an elastomeric diaphragm dividing the interior of the housing into two distinct fluid chambers. The piston type brake actuator is substantially similar to the diaphragm type, except that instead of a diaphragm, a piston reciprocates in a cylinder for applying the normal and/or parking brakes of the vehicle.
In a typical service brake actuator, the service brake housing is divided into a pressure chamber and a pushrod chamber. The pressure chamber is fluidly connected to a source of pressurized air and the pushrod chamber mounts a pushrod that is coupled to the brake assembly. The introduction and exhaustion of pressurized air in to and out of the pressurized chamber reciprocates the pushrod in to and out of the housing to apply and release the operating brakes.
In a typical spring brake actuator, the spring brake section is divided into a pressure chamber and a spring chamber by a diaphragm. A pressure plate is positioned in the spring chamber between the diaphragm and a strong compression spring, whose opposing end abuts the housing of the section. In one well-known configuration, an actuator tube extends through the pressure plate, through the diaphragm, into the pressure chamber, and through a dividing wall separating the spring brake actuator from the service brake actuator. The end of the actuator tube is fluidly connected to the pressure chamber of the service brake actuator.
When applying the parking brakes, the spring brake actuator pressure is discharged from the pressure chamber and the large force compression spring pushes the pressure plate and the diaphragm toward the dividing wall between the spring brake actuator and the service brake actuator. In this position, the actuator tube connected to the pressure plate is pushed for applying the parking or emergency brakes and thus immobilizing the vehicle. To release the parking brake, pressurized air is introduced into the pressure chamber of the spring brake actuator to expand the pressure chamber, move the diaphragm and pressure plate toward the opposing end of the spring brake actuator housing, and compress the compression spring.
One known problem in association with spring brake actuators of this design is that as the large force compression spring is compressed, the pressure chamber increases in volume and the spring chamber decreases in volume, resulting in a pressure increase in the spring chamber. The build-up of pressure in the spring chamber upon the release of the brake is highly undesirable in that any pressure build-up in the spring chamber must be offset by an increased pressure in the pressure chamber in order to fully compress the spring and thus fully release the brake.
The undesirable effects of pressure build-up in the spring chamber are exacerbated due to the fact that most pressurized air systems for heavy-duty vehicles operate at an industry standard maximum pressure. If the combined pressure of the spring and the air pressure in the spring chamber approach that maximum pressure then the emergency brake can fail to release, only partially release, or release very slowly, all of which are undesirable.
One solution to prevent pressure increase in the spring chamber is to include vent holes in the spring chamber housing. These vent holes are undesirable because they expose the interior of the spring chamber to external environmental elements such as dirt, salt, and water, which accelerate abrasion, corrosion, or wear on the various internal brake components such as the spring. The damage to the internal brake components by environmental elements can require increased maintenance or cause premature failure of the spring. To prevent environmental elements from entering the spring brake housing, it is known to place a filter over the vent openings. The filtered vent openings, however, inherently permit external air to enter the brake, yielding a brake that is not completely sealed. Additionally, the filters require increased maintenance because they must be cleaned and/or replaced and typically do not effectively prevent water from entering the spring chamber.
An additional problem with directly externally venting the spring chamber is that the types of vehicles on which the actuator is mounted, such as tractor trailers, are often parked for extended periods in a dock bay. The bays are typically sloped and below grade, and under heavy rain or snow conditions can fill with water to a height that floods the interior of the actuator's spring chamber. Although the water is normally expelled from the spring chamber through the vent openings as the brake is released, the flooding can accelerate corrosion and introduce other environmental hazards. Further, if it is below freezing, the water can freeze and prevent release of the brake. Filtered vent openings do not prevent water from flooding the spring chamber.
In order to eliminate the pressure build-up in the spring chamber while keeping out environmental elements, it is known to include a fluid flow path between the spring chamber of the spring brake actuator and the service brake pressure chamber through the actuator tube. In such an actuator, a control valve is placed in the actuator tube to regulate air flow between the spring chamber and service brake pressure chamber. Two types of control valves have been used, two-way control valves and one-way control valves.
One-way control valves allow air to flow from the spring chamber into the service brake pressure chamber to prevent pressure build up in the spring chamber when the volume of the spring chamber decreases. However, when the spring brake is applied and the volume of the spring chamber increases, the one-way valve remains in its closed position and does not allow air to flow from the service brake pressure chamber into the expanding spring chamber. This causes a vacuum to form in the spring chamber such that volume enclosed by the spring chamber is at a negative relative pressure, which reduces the load provided by the parking brakes. In order to overcome the vacuum formation in the spring chamber, it is necessary to use a larger spring in the spring chamber that is capable of overcoming the negative relative pressure caused by the vacuum. While pneumatic brake actuators with one-way control valves typically allow air to enter the spring chamber through the opening that the caging bolt passes through when the spring is caged, this opening is sealed during normal operation of the actuator.
Two-way control valves allow air to vent from the spring chamber when the spring is compressed and they allow air to enter the spring chamber when the spring brake is applied. Conventional two-way control valves, however, are flow rate sensitive, which means that they will only close due to increased pressure in the service brake pressure chamber if the flow of fluid into the service brake pressure chamber and through the valve reaches a certain level. If fluid slowly moves through the valve while the service brake pressure chamber increases, the valve remains open, thereby allowing the pressure to increase in both the service brake pressure chamber and the spring chamber. This results in application of both the service brake and the spring brake which subjects the brake system's components to increased levels of force that may cause damage.